We have continued our studies on the genes involved in cellular commitment of the myoblast and its subsequent differentiation during myogenesis. The precursor myoblast is defined early in development within the somite compartment as a group of cells expressing the MyoD family of gene regulatory factors, MyoD, myogenin, myf5, and MRF4. The expression of these factors not only marks the committed cell but also plays a role in activating the muscle-specific genes during terminal differentiation. We have isolated these four genes in the chicken and have been studying their pattern of expression in development, both in vivo and in vitro, determining the role of phosphorylation in their functional regulation, and identifying other related bHLH proteins that interact with these myogenic factors to regulate their function. In order to take a genetic approach to the study of myogenesis we have also isolated the MyoD homolog in Drosophila where it is a single gene that marks a subset of muscle precursor cells. By studying the Drosophila gene, the proteins that regulate its function, and the upstream and downstream genes in the myogenic pathway in the fly, we hope to identify correlates in the vertebrates to help us understand more completely vertebrate myogenesis.